My cooker hood needs replacing and the current ducting has a diameter of about 110 mm (external measurement). However, the usual ducting diameter for cooker hoods seems to be 100, 125 and 150 mm. Am I correct to to take an external measurement and Is it possible to find a reducer or adapter to allow fitting?
It sounds like you have either external flexible underground duct or dust extraction hose as this size is a common 110-114mm. Sounds like the builder had some spare. Remember that Cooker hood duct should be of a fire resistant material for obvious reasons.